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Lightning Simulation of a Combined Overhead Line/Cable Connected GIS

by

Jakob Kessel, Víðir Már Atlason and Claus Leth BakAalborg University,

Institute of Energy Technology

and

Jesper LundNV Net A/S

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Introduction

∙ 170 kV transmission system for year 2014▫ Mainly underground cable and GIS

∙ Follow up on 9th semester project

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Introduction

∙ Only Area 3 showed overvoltages

∙ Modelling▫ Lines, cables and outdoor busbars

◦ Transmission lines

▫ GIS busbars and transformers ◦ Equivalent capacitances

▫ Tower model▫ Grounding resistance▫ Surge arrester

SA

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Modelling

∙ Tower model▫ Tower surge impedance▫ Insulator model▫ Grounding resistance

Cite: Fast Front Task Force of the IEEE, and Sargent et al.

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Modelling

∙ Insulator model▫ Voltage-time characteristic

75,02

1tv tK

KU

Cite: Fast Front Task Force of the IEEE, and Yadee et al.

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Modelling

∙ Grounding resistance▫ Dynamic grounding resistance

▫ Where:◦ R0 is low current grounding resistance

◦ Ig is the critical current causing ionization of the soil

◦ IR is the current to ground

∙ Surge arrester model ▫ Simplified IEEE model

Cite: Fast Front Task Force of the IEEE, and Crisholm et al.

Cite: Pinceti et al.

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Simulation

∙ Simulation parameters▫ Shielding failure▫ Back flashover

▫ The lightning surge is estimated with double exponential function

∙ Parameter investigation▫ Lightning front time

◦ Only for shielding failure

▫ Soil resistivity (at the overhead line/cable transition point)▫ Cable length (between transformer and surge arrester in GIS)

Front time Time to half Crest magnitude Soil resitivity

[µs] [µs] [kA] [Ωm]

Shielding failure 1,4 350 -41,8 92,5

Back flashover 10 350 -200 92,5

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Results

Utf

∙ Simulation results ▫ Varying lightning

front time, SF, closed breaker

▫ Varying lightningfront time, SF, open breaker

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Results

∙ Simulation results▫ Varying soil

resistivity, SF, closed breaker

▫ Varying soil resistivity, SF, open breaker

Utf

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Results

∙ Simulation results▫ Varying cable

length, SF, closed breaker

▫ Varying cable length, SF, open breaker

Utf

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Results

∙ Evaluation of critical lightning current▫ Varying front time

◦ Only evaluated for shielding failure◦ No overvoltages with closed breaker

▫ Varying soil resistivity◦ No overvoltage for SF with closed breaker

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Results

∙ Evaluation of critical lightning current▫ Cable length

◦ No overvoltages with closed breaker

▫ The MTBF is found based on the lightning current

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Modelling

∙ Risk assesment▫ Back flashover

◦ MTBFclosed = (P(closed) P(current) Nflashes)-1

◦ MTBFopen = (P(open) P(current) Nflashes)-1

◦ P(open) ≈ 1/365

◦ P(closed) = 1 - P(open)

▫ Shielding failure◦ MTBFclosed = (P(closed) P(sf) P(current) Nflashes)-1

◦ MTBFopen = (P(open) P(sf) P(current) Nflashes)-1

∙ Mean Time Between Failure

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Conclusion & Discussion

∙ Conclusion

▫ The steepness of the lightning surgeLimited effect on the overvoltages.

▫ The soil resistivity at the overhead line/cable transition pointGreat effect on the overvoltages.

▫ The cable length between the transformer and the surge arrester in the GISIncreased cable length yielding increased voltage magnitude at the transformer.

▫ MTBF > 2000 yearsThe surge arrester at the overhead line/cable transition point provides adequate protection for the substation.Further protection in form of a surge arrester at the GIS busbar is not necessary.